Non-destructive inspection (NDI), also referred to as non-destructive evaluation (NDE) and non-destructive testing (NDT), of structures involves thoroughly examining a structure without harming the structure or requiring its significant disassembly. Non-destructive inspection is typically preferred to avoid the schedule, labor, and costs associated with removal of a part for inspection, as well as avoidance of the potential for damaging the structure. Non-destructive inspection is advantageous for many applications in which a thorough inspection of the exterior and/or interior of a structure is required. For example, non-destructive inspection is commonly used in the aircraft industry to inspect aircraft structures for any type of internal or external damage to or flaws in the structure. Inspection may be performed during manufacturing or after the completed structure has been put into, including field testing, to validate the integrity and fitness of the structure. In the field, access to the interior surfaces of the structure is often restricted, requiring disassembly of the structure, introducing additional time and labor.
Among the structures that are routinely non-destructively tested are composite structures, such as composite sandwich structures and other adhesive bonded panels and assemblies including carbon fiber or graphite reinforced epoxy (Gr/Ep) and non-ferromagnetic metals (e.g. aluminum alloy, titanium alloy, or aluminum or titanium hybrid laminates such as GLARE or Ti/Gr). In this regard, composite structures are commonly used throughout the aircraft industry because of engineering qualities, design flexibility, low weight, and high stiffness-to-weight ratio. A shift toward bonded materials dictates that devices and processes are available to ensure structural integrity, production quality, and life-cycle support for safe and reliable use. As such, it is frequently desirable to inspect composite structures to identify any flaws, such as cracks, voids or porosity, which could adversely affect performance. For example, typical flaws in composite sandwich structures, generally made of one or more layers of lightweight honeycomb or foam core material with composite or metal skins bonded to each side of the core, include disbonds which occur at the interfaces between the core and the skin or between the core and a buried septum.
Various types of sensors may be used to perform non-destructive inspection. One or more sensors may move over the portion of the structure to be examined, and receive data regarding the structure. For example, a pulse-echo (PE), through transmission (TT), or shear wave sensor may be used to obtain ultrasonic data, such as for thickness gauging, detection of laminar defects and porosity, and/or crack detection in the structure. Resonance, pulse echo or mechanical impedance sensors may be used to provide indications of voids or porosity, such as in adhesive bondlines of the structure. High resolution inspection of aircraft structure is commonly performed using semi-automated ultrasonic testing (UT) to provide a plan view image of the part or structure under inspection. While solid laminates may be inspected using one-sided pulse echo ultrasonic (PEU) testing, composite sandwich structures typically require through-transmission ultrasonic (TTU) testing for high resolution inspection. In through-transmission ultrasonic inspection, ultrasonic sensors such as transducers, or a transducer and a receiver sensor, are positioned facing each other but contacting opposite sides of the structure. An ultrasonic signal is transmitted by at least one transducer, propagated through the structure, and received by the other transducer. Data acquired by the sensors is typically processed and then presented to a user via a display as a graph of amplitude of the received signal. To increase the rate at which the inspection of a structure is conducted, a scanning system may include arrays of inspection sensors, i.e., arrays of transmitters and detectors. As such, the inspection of the structure can proceed more rapidly and efficiently, thereby reducing the costs associated with the inspection.
Many structures are difficult to accurately inspect using PE or TTU scanning. X-ray inspection may be preferred for certain situations because of the high flaw resolution and ability to image flaws that are not parallel to the surface and without the use of a couplant. X-ray inspection could be used for close-out inspection of bonded wings, spar e-beams, complex composite sandwich structures, complex laminate composites, titanium graphite (Ti/Gr) materials, and fiber-reinforced metal-matrix composites (MMCs). Conventional x-ray inspection systems expose film that is developed to create an image that can be analyzed visually. Recently, CCD (charge coupled device) and CMOS (complementary metal oxide semiconductor) detectors have been used for the imaging, rather than film. Traditional 2-D inspection techniques, however, produce a projection of 3-D space into 2-D; thus, objects are superimposed and depth information is lost. And although ultrasonic pulse-echo inspection is able to retain depth information, near-surface features and flaws can mask other features and flaws further from the surface of the structure. In addition, the image resolution obtained using ultrasonic pulse-echo inspection may not be high enough for some applications, and flaws parallel to the inspection signals may be difficult to find and identify.
Non-destructive inspection may be performed manually by technicians who typically move an appropriate sensor over the structure. Manual scanning generally consists of a trained technician holding a sensor and moving the sensor along the structure to ensure the sensor is capable of testing all desired portions of the structure. However, typical x-ray inspection applications operate with high power emissions which prevent manual operation.
Semi-automated inspection systems have been developed to overcome some of the shortcomings with manual inspection techniques. For example, the Mobile Automated Scanner (MAUS®) system is a mobile scanning system that generally employs a fixed frame and one or more automated scanning heads typically adapted for ultrasonic inspection. A MAUS system may be used with pulse-echo, shear wave, and through-transmission sensors. The fixed frame may be attached to a surface of a structure to be inspected by vacuum suction cups, magnets, or like affixation methods. Smaller MAUS systems may be portable units manually moved over the surface of a structure by a technician. However, for through-transmission ultrasonic inspection and x-ray inspection, a semi-automated inspection system requires access to both sides or surfaces of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for semi-automated systems that use a fixed frame for control of automated scan heads.
Automated inspection systems have also been developed to overcome the myriad of shortcomings with manual inspection techniques. For example, the Automated Ultrasonic Scanning System (AUSS®) system is a complex mechanical scanning system that employs through-transmission ultrasonic inspection. The AUSS system can also perform pulse echo inspections, and simultaneous dual frequency inspections. The AUSS system has robotically controlled probe arms that must be positioned proximate the opposed surfaces of the structure undergoing inspection with one probe arm moving an ultrasonic transmitter along one surface of the structure, and the other probe arm correspondingly moving an ultrasonic receiver along the opposed surface of the structure. Another example robotic system is the x-ray inspection system used at the William-Gateway Structural Repair Facility in Mesa, Ariz., for inspection of F-18 tail sections. Conventional automated scanning systems, such as the AUSS-X system and the William-Gateway x-ray system, therefore require access to both sides or surfaces of a structure which, at least in some circumstances, will be problematic, if not impossible, particularly for very large or small structures. To maintain the transmitter and receiver in proper alignment and spacing with one another and with the structure undergoing inspection, the AUSS-X system has a complex positioning system that provides motion control in ten axes.
Conventional x-ray inspection systems are gantry systems. Many parts, however, are too large for examination; the system cannot reach the full extent of the part because the scan envelope of the system is limited. Further, conventional NDI systems are not designed to provide detailed information about internal features of a structure, particularly with respect to internal damage or flaws that are parallel to an inspection signal.